Speedwell of Rhu’s original owner planned to take her across the Atlantic so he had her fitted with 4 water tanks (and 2 diesel tanks). We’ve had one of the water tanks converted to a holding tank.
Water Tanks
Each of the now three water tanks has its own deck filler, isolation valve, inspection hatch and dip-stick. The tanks are made from GRP, which gives a solid tank (on our previous boat the water tanks were PVC and developed splits).
Our surveyor recommended carrying drinking water in separate containers because of the possibility of osmosis developing in GRP water tanks. Clearly that would be inconvenient so we looked at options for ensuring that the water from our tanks is good to drink. Lining the existing tanks was an obvious solution, but that would have involved quite a lot of work and would have made it difficult to add contents gauges.
After some research we decided to fit a Seagull water filter to the drinking water supply. Seagull’s manufacturer has assured us that the filter is able to remove any osmosis products that may be in the water, making it safe to drink. Another advantage is that this make of filter removes microorganisms so if we do pick up some dodgy water we’ll still have a safe drinking supply. The filtered water certainly tastes better than water directly from the tanks, most notably there’s no chlorine flavour, so it’s certainly removing something.
The Seagull filter feeds a dedicated drinking water tap in the galley. Water for washing up etc. comes straight out of the tanks.
Our three water tanks have a combined capacity of a little over 600 litres, two of us can live aboard for three or four weeks on a full set of tanks without being too caution about water use. Before making ocean passages we will fit a salt water tap in the galley to help to reduce fresh water usage and will consider fitting a water maker.
Holding Tank
A sewerage holding tank is becoming a requirement in some places and in any case means that we can use the heads while we’re in a marina without worrying about discharging sewerage in a confined area.
We considered various options, including sacrificing a locker for a new tank, but finally decided to convert one of the aft water tanks to a holding tank. The remaining three tanks still provide plenty of fresh water capacity, especially as we normally only have two people on board. The holding tank is plumbed in to the aft heads, which are accessible from all parts of the boat, when we’re in mandatory holding tank areas we close off the forward heads (we normally only use the forward heads compartment as a shower cubicle anyway).
We’ve got two options to empty the tank; a deck pump-out and rinse for use at pump-out stations and a pump to the heads outlet for use at sea. We decided to go for a manual pump rather than an electric macerator, largely because it’s simpler and so should be more reliable. We have the same model pump for tank pump out as it fitted to the Lavac toilets, so we’ve got common spares and also anything that got into the tank went through one of these pumps so it should go out through another one! Incidentally, the Lavac toilets are proving to be reliable and simple to use and don’t seem to suffer from bad smells that can afflict dual action pump toilets.
Fuel Tanks
Speedwell has two diesel tanks giving a total capacity of 450 litres. Motoring at 6 knots we burn around 3.5 litres an hour so we have a motoring range of over 700 miles, by which time the engine is due for a 100 hour service.
The engine fuel pick-up is switchable between tanks but the return only goes to the main tank so when we’re running from the reserve fuel is gradually transferred over to the main tank. This system took a little getting used to, but the transfer rate is quite slow and with the Tank Tender able to accurately measure each tank’s capacity it’s not too difficult to manage.
Our diesel powered heater takes its fuel from the reserve tank.
Contents Gauges
When we bought Speedwell she had no contents gauges on any of her tanks There was a dipstick for each tank, a completely reliable but rather inconvenient measurement method. We decided to fit a Tank Tender based on its simplicity, absence of moving parts in the tanks and ability to measure the contents of any type of tank. This is set up to measure the fuel and water tanks, a separate Wema gauge system measures the holding tank contents.
The Tank Tender reads the depth of liquid in the tank. We’ve got calibration tables for each tank that were generated by filling the tanks in measured steps and recording the depth reading at each step. This gives us an accurate measure of current contents allowing for the shape of each tank (much more accurate than a traditional contents gauge).
In practice we use the forward and aft water tanks as reserve capacity so they are each either full or empty (having drained into the central water tank when the isolation valve was opened). We could have got away with a system just reading the main water tank and the two diesel tanks.